1. Field of the Invention
The present invention relates generally to electrohydraulic servovalves and more particularly to such valves which include improved feedback loops which provide higher rates of motion of a load to a commanded position and at the same time provide damping to cushion forces which are applied to the load. Although not limited thereto the electrohydraulic servovalve of the present invention is particularly useful in the field of animatronics. The field of animatronics is the simulation of living beings by various combinations of mechanical, electrical and hydraulic means.
2. Description of the Prior Art
Various control systems for manipulator type apparatus have been proposed and/or implemented utilizing servo loops having command signals and feedback signals to position a particular apparatus such as a manipulator arm on a robotic device or the like. Such devices are utilized for various types of work such as the positioning of a work-piece into a receiving device or the like.
The term "compliance" as used throughout the specification and claims is defined to mean the use of a low-authority force-related feedback term in addition to the traditional position feedback in an actuator loop. The force feedback is generally such that forces or inertial loads applied to the system cause the opening of a hydraulic valve which tends to cushion such forces (rather than opposing them) while the position loop is simultaneously opposing them to achieve or maintain the commanded position. Such compliance is particularly useful in animatronics. The purposes of compliance in such animatronics applications are to allow higher rates of motion with high inertia moves (such as a shoulder driving an extended arm) without destroying the figure's structure with the inertial loads due to sudden starts and stops; to provide dynamic damping which avoids the "bounce" appearance of an under damped move; and to provide a more natural look to the overall animated character where other unconnected moves in the same structure (such as the elbow) react to accelerations and decelerations without specific position commands to do so. The overall objective in animatronics is for the figure's movement to look as natural as possible.
The prior art known to applicant which utilizes the term compliance therein typically deals only with reducing the apparent stiffness of a hydraulically or electromechanically driven device to an externally applied force. This is normally done to allow such apparatus, such as robot manipulators, to do mechanical assembly work which requires reduced stiffness to allow for variation in work-piece and positional placement. If such is not done then an extremely stiff servo system will force the work-piece into the receiving device and if the two are not exactly aligned the work-piece may be damaged. A typical example of such a structure is illustrated in U.S. Pat. No. 5,206,930 to Ishakawa et al. In this structure compliance control is in response to externally applied forces and is achieved through force feedback fed into a control algorythym. The control system allows the mechanical stiffness in any axis to be specified as a commanded input. It should be noted that this and other such manipulator systems, for example, that as illustrated in U.S. Pat. No. 4,792,715 to Barsky are directed to stiffness resulting from externally applied forces and not to inertial stiffness. The Barsky patent is furthermore directed to controlling force only rather than primarily controlling position with a force feedback component.
Compliance as presently known in the prior art animatronics systems is generally accomplished by placing load cells in series with the actuator linkages to provide force feedback signals. Such a structure produces significant mechanical packaging disadvantages in that the space available in the animatronic figure for receiving these structures is usually extremely limited. For example a longer actuator package is required to incorporate the transducer such as the load cell and furthermore there is a significant increase in the wire count for the force transducer leads. In addition to the foregoing, typical current prior art compliant systems have a steady state positional error resulting from a static load being applied to the structure. An example of such a case is where the actuator is required to hold up the weight of a limb or an extended neck or head such as would occur in a dinosaur type animated structure. Usually the animatronic figures of this type are programmed for position with the compliance turned off; this then requires re-tuning of the programming for positional accuracy when the compliance is turned on. Such requires additional expense and if the load is somewhat increased for any particular movement then the result would be an undesirable movement of the figure.
Other prior art known to applicant which relates generally to this field is shown in U.S. Pat. Nos. 4,510,428, 4,164,167, 4,031,813, 4,826,110, 4,598,626, 4,712,470, 5,206,569, 3,763,746, Russian Patent 757777 and Japanese Patent Application No. 2-263127 filed Oct. 2, 1990 entitled Compliance Control Method.